It is known to control the injection of fuel in each cylinder of a Diesel engine using an open-loop control. In these conventional systems the injection time and the pulse width of the injections are selected from predefined values stored in the engine's electronic control unit (ECU), on the base of one or more engine characteristic parameters, such as for example engine speed, engine load, environ-mental temperature and pressure.
Although such systems exhibit acceptable performance, they show typical defects and drawbacks of open-loop control. For example, the flow characteristics of an injector in a Diesel engine may change during time as a result of aging phenomena, thus the pulse width used for the injector will no longer supply the cylinder with the desired quantity of fuel, and in general the performance of the engine will be degraded, giving way to higher polluting emissions, higher fuel consumption, increased noise and even the possibility of damage to the engine.
In order to improve such situation, recent Diesel engine systems provides a closed-loop control of a parameter representative of the fuel combustion in the engine cylinders, in order to stabilize the combustion and reduce polluting emission.
One of the mostly used parameter in controlling the combustion of a Diesel engine is the position of MFB50, which is the crank angle in which the 50% of mass of the fuel injected into the cylinder has been burnt.
The determination of said parameter requires the ECU to sample the pressure within the cylinder during an engine cycle, to thereby determining an in-cylinder pressure curve; to use said in-cylinder pressure curve for calculating a heat release curve over the same engine cycle; and finally to calculate the position of MFB50 on the base of said heat release curve.
The pressure is sampled by means of a pressure sensor set inside the cylinder and typically integrated in the glow plug associated to the cylinder itself; such pressure sensor being connected with the ECU via an analog/digital converter.
In order to achieve the best control accuracy, have been considered Diesel engine control systems configured for controlling the combustion in each cylinder of the engine, independently and contemporaneously.
As a matter of fact, such control systems require at least one pres-sure sensor per cylinder, one analog/digital converter per pressure sensor, and an ECU having an elevated throughput/computational capacity, to thereby contemporaneously sampling the in-cylinder pressure curve of each cylinder. One serious drawback of such control systems is therefore the high cost of the electronic components, in particular of pressure sensors, analog/digital converters and ECU.
In order to reduce total cost, Diesel engine control systems have been configured for using one pressure sensor only, in order to sample the pressure within a single “lead cylinder” of the engine, and for applying information derived from such pressure sensor to control also the cylinders without pressure sensor. As a matter of fact, this second approach gives way to a closed-loop control of “lead cylinder” with pressure sensors and subordinated open-loop control of non-sensed cylinders depending on “lead cylinder”. This second approach is performed under the assumption that the pres-sure curves are substantially identical across the different cylinders.
However, has been shown that in a Diesel engine there may be one or more cylinders in which the combustion takes place in different way than in the other cylinders, resulting in different in-cylinder pres-sure curve. Such different behavior can depend for example on the geometry of the intake manifold, whose design can be such that, under certain engine operating conditions, one or more cylinders receive different quantities of EGR.
It follows that the in-cylinder pressure curve sampled in a single cylinder is not always properly representative of the combustion in the other cylinders, and consequently that the control of non-sensed cylinders has a limited quality, leading to increased fuel consumption, polluting emission, etc.
At least one object of the present invention is to provide an in-cylinder pres-sure curve which takes into account more than one cylinder, in order to be suitable for performing a closed-loop control having better quality than that based on the in-cylinder pressure curve sampled in a single cylinder. At least another object of the present invention is to provide an in-cylinder pressure curve whose determination requires reduced hardware re-sources and involves reduced throughput and/or computational load for the ECU than that necessary for sampling the in-cylinder pressure curve in each cylinder, to thereby reducing the total cost of the engine system. In addition, other objects, desirable features, and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.